Various arrangements are known for overlaying graphic images over a video signal received from a separate remote source as, for example, a broadcast television system, a video disk, or a video tape recorder. These graphic images can be, for example, alphanumeric characters for display on a television monitor also displaying the video picture as derived from the received video signal.
U.S. Pat. No. 4,639,765 (D'Hont), issued on Jan. 27, 1987, discloses a synchronization system for overlaying an internal video signal upon an external video signal. Phase synchronization of the internal video signal with the external video signal is achieved by generating a video clock signal having a frequency which is a predetermined multiple of the horizontal line rate of the external video signal. This video clock enables generation of horizontal lines which have a predetermined phase relation to the external video signals. Still further, the frames of the internal video signal are phase synchronized with the frames of the external video signal. This is achieved by detecting the vertical reset pulses of the external video signal and resetting the video generator when such external video signal vertical reset pulses are received. Loss of the external video signal causes the system to operate in an "internal only" mode until the external video signal is again received. When reacquiring the external video signal, the system re-synchronizes to the external video signal for a return to an external mode of operation.
U.S. Pat. No. 5,051,817 (Takano), issued on Sep. 24, 1991, discloses a system for superimposing color characters on an input video signal. In the system, a first Sync Separator separates horizontal sync pulses from the input video signal. These horizontal sync pulses are used by a Phase Lock Loop (PLL) circuit to generate a reference clock signal (P1) that is locked to the horizontal sync pulses of the input video signal. A second Sync Separator, a Timing Generator, a Burst Gate, and a second PLL circuit generate an oscillation output signal that is phase locked to a burst signal of the input video signal. The reference clock signal and the oscillation output signal are used to synchronize a generated character signal with the input video signal. A Changeover Signal Generator generates changeover control signals to either deliver just the input video signal or the input video signal superimposed with color characters to an output of the system.
Color in, for example, an National Television Systems Committee (NTSC) video signal is conveyed in a color subcarrier on the video signal, while brightness is conveyed through the main signal. The NTSC video signal is an analog signal wherein the color information is contained in the phase and amplitude of the color subcarrier whose reference is contained in a short color burst at the beginning of video information for each line in the television picture. The color burst allows the television receiver to look at the burst, which is a small snapshot of the color subcarrier, and lock onto the color subcarrier. Present Cable Television (CATV) Converter boxes are analog converters which provide an On Screen Display (0SD) as for example, the number of the presently tuned television channel being viewed, etc. It is necessary to look at the color burst to overlay the right subcarrier phase onto the characters being generated for the OSD since the phase of the color subcarrier directly imparts information as to the color of the character.
In the near future it is expected that Cable television (CATV) companies will be transmitting digital TV signal possibly mixed with analog TV signals. With digital TV transmissions, "1's" and "0's" are transmitted rather than an analog video signal. Digital TV transmissions provide clearer pictures since interference is avoided. To be compatible with current TV receivers, the digital TV signal has to be converted to a standard analog TV signal. At an exemplary digital Cable TV Converter, the color subcarrier is locally generated and used to generate an analog TV signal. More particularly, the Cable TV Converter comprises a Video Encoder that receives the digital TV signal which does not have color subcarrier information in it. Instead, the digital TV signal is completely in a component form including, for example, Red, Green, and Blue color information. Therefore, a Digital Video Encoder in a digital terminal or in future Cable TV Converter must recreate the color subcarrier in order for the TV receiver to reproduce the desired picture. That same locally generated color subcarrier can be easily used to process digital character information and precisely overlay the digital characters on the digitally received TV signals.
A problem is that the digital Video Encoder used with the digital TV signals is not locked onto analog TV signals that are also received on the same input cable line. Therefore, it is desirable to provide a digital TV receiving device wherein a Digital Video Encoder is usable with received analog TV signals, and has the ability to overlay digital characters with proper color on the analog TV signals without causing On Screen Display jitter.